Breathing at High Altitude

Everyone breathes harder and deeper at altitude

Blood is oxygenated as it passes through the lungs

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Everyone breathes faster and deeper (hyperventilates) at high altitude – it is necessary to do this in order to survive. The function of the lungs is to expose blood to fresh air, and breathing faster essentially increases the flow of fresh air past the blood. This means that whenever an oxygen molecule is taken away by the blood, it is quickly replaced by a fresh one. This means that there is always more oxygen available to be taken up into the blood. (Click to read more about the carriage of oxygen in the blood.)

Carbon dioxide (CO2) is constantly produced by the body and the lungs remove it by allowing it to diffuse into the fresh air in the lungs. Increasing the flow of fresh air through the lungs, by hyperventilating, increases the rate at which CO2 is lost. This happens for the same reason that wet laundry dries faster in a strong wind: the wind blows away the water vapour, so there is space for more water to evaporate. You can see how hyperventilating changes the level of carbon dioxide in the blood using the altitude oxygen calculator. Simply increase the number of breaths taken per minute, and watch what happens to the CO2.

Because CO2 is an acid gas, losing more of it from the blood leaves the blood relatively alkaline. At altitudes up to about 6000m, the kidneys correct the alkalinity of the blood over a few days by removing alkali (in the form of bicarbonate ions, HCO3-) from the blood. Our oxygen calculator will allow you to remove bicarbonate ions; watch the effect on the alkalinity of the blood.

These processes have important effects on the binding of oxygen to haemoglobin in the blood. You can read more about this in our tutorial on oxygen transport.

The diagrams on this page show what happens if there is a blockage to an air space in the lungs. The simplest example is a peanut stuck in one of the air passages in the lungs, but the same process happens in pneumonia, or pulmonary edema. Blood still flows past the air spaces, but because there is no fresh air getting to the blood, it can’t take up any more oxygen. That means that a lot of de-oxygenated blood makes its way straight past the lungs. When it mixes with the blood from all the other parts of the lungs, the total amount of oxygen is less. This causes hypoxia – a shortage of oxygen getting to where it is needed.

Physiological Shunt

Even in completely healthy people, there is always some blood that makes its way past the lungs without encountering any fresh air. This blood passes through the bronchial circulation, and the thebesian veins in the heart. This is called physiological shunt. You can calculate the effect of different amounts of shunt by downloading our oxygen delivery model in microsoft excel (but beware – this is a very large file).

How do the lungs cope with shunt?

We have evolved a clever mechanism to reduce the effect of shunt. When the blood passing through an area of lung isn’t picking up enough oxygen, the blood vessels carrying that blood tighten, so that less deoxygenated blood can get through the lungs. This is called hypoxic pulmonary vasoconstriction. The diagram to the right shows how this means that less deoxygenated blood gets through, so there is more oxygen in the mixture of blood leaving the lungs.

Even in completely healthy people, there is always some blood that makes its way past the lungs without encountering any fresh air. This blood passes through the bronchial circulation, and the thebesian veins in the heart. This is called physiological shunt. You can calculate the effect of different amounts of shunt by downloading our oxygen delivery model in microsoft excel (but beware – this is a very large file).

How do the lungs cope with shunt?

We have evolved a clever mechanism to reduce the effect of shunt. When the blood passing through an area of lung isn’t picking up enough oxygen, the blood vessels carrying that blood tighten, so that less deoxygenated blood can get through the lungs. This is called hypoxic pulmonary vasoconstriction. The diagram to the right shows how this means that less deoxygenated blood gets through, so there is more oxygen in the mixture of blood leaving the lungs.

At high altitude, there is less oxygen in the air that you breathe. This means that all of the blood from all areas of the lungs, is relatively short on oxygen or hypoxic.

Unfortunately, the lungs still respond to the shortage of oxygen in the same way: by tightening the blood vessels. But because all areas of the lung are lacking in oxygen, all of the blood vessels in the lungs constrict.

The heart still pumps the same amount of blood through the lungs, but because all of the blood vessels are tightly constricted, the pressure needed to force blood through them is much greater. In fact the pressures get so high that some of the tiniest blood vessels break open, and this is thought to be part of the cause of HAPE (high altitude pulmonary edema).